Impact of π‑Expanded Boron-Carbonyl Hybrid Acceptors on TADF Properties: Controlling Local Triplet Excited States and Unusual Emission Tuning

Three
donor–acceptor-type thermally activated delayed fluorescence
(TADF) emitters (PXZBAO (1), PXZBTO (2),
and PXZBPO (3)) comprising a phenoxazine (PXZ) donor
and differently π-expanded boron-carbonyl (BCO) hybrid acceptor
units are proposed. The emitters exhibit red (1) to orange
(3) emissions with an increase in the π-expansion
in the BCO acceptors. The control of the strength of local aromaticity
for the BCO unit and the corresponding LUMO level is attributed to
inducing the unusual emission color shifts. The photoluminescence
quantum yield and delayed fluorescence lifetime of the emitters are
also adjusted by the π-expansion. Notably, although 1 possesses a 3nπ* state in the acceptor unit as
a local triplet excited state (3LE, T2), the
T2 states of 2 and 3 mainly comprise
a 3ππ* state in the acceptor. Consequently,
all of the emitters exhibit strong spin–orbit coupling between
their T2 and excited singlet (S1) states, leading
to a fast reverse intersystem crossing with rate constants of ∼106 s–1. By employing the emitters as dopants,
we realize efficient red-to-orange TADF-OLEDs. Maximum external quantum
efficiencies of 17.7% for the yellowish-orange (3), 15.5%
for the orange (2), and 13.9% for the red (1) devices are achieved, and the values are very close to the theoretical
limit predicted from the optical simulation.